Metal-oxide-semiconductor field effect transistors, often referred to as MOSFETS, are widely used in electronic devices, such as for switching or amplification. MOSFETS are capable of achieving fast switching speeds, which makes them well suited for high-frequency applications. Additionally, MOSFETS are relatively simple to control because they are voltage-controlled, not current-controlled.
Lateral double-diffused metal-oxide-semiconductor field effect transistors, often referred to as LDMOS transistors, are a class of MOSFETS where drain-to-source voltage is blocked within the transistors' semiconductor material primarily in a lateral direction. LDMOS transistors are often combined with other circuitry in integrated circuits, especially in power and radio-frequency applications.
FIG. 1 is a cross-sectional view of a prior art n-channel LDMOS transistor 100 including a silicon semiconductor structure 102, a source electrode 104, a gate structure 106, and a drain electrode 108. Source electrode 104 is stacked on a top surface 110 of silicon semiconductor structure 102 in a source region 112 of LDMOS transistor 100, and drain electrode 108 is stacked on top surface 110 in a drain region 114 of LDMOS transistor 100. Gate structure 106 includes a gate electrode 116, a polysilicon layer 117, and a silicon dioxide layer 118 stacked in a gate region 120 of LDMOS transistor 100. Silicon semiconductor structure 102 includes a p-type substrate 122, an n-well 124, a p-body 126, a source p+ region 128, a source n+ region 130, and a drain n+ region 132. N-well 124 is formed over p-type substrate 122, and p-body 126 is formed in n-well 124 under source electrode 104. Drain n+ region 132 is formed in n-well 124 and contacts drain electrode 108. Each of source p+ region 128 and source n+ region 130 is formed in p-body 126 and contacts source electrode 104. Each of source n+ region 130 and drain n+ region 132 is more heavily doped than n-well 124, and source p+ region 128 is more heavily doped than p-body 126.
When positive voltage VDS is applied across drain electrode 108 and source electrode 104, a p-n junction at the interface of n-well 124 and p-body 126 is reversed biased. Consequentially, essentially no current flows from drain electrode 108 to source electrode 104 by default. The relative dopant concentration of drain n+ region 132 and n-well 124 causes a portion of n-well 124—referred to as a drift region 134—to carry the majority of voltage VDS, thereby enabling LDMOS transistor 100 to support a relatively large value of VDS without breakdown.
A positive voltage VGS applied between gate electrode 116 and source electrode 104 creates negative charges in silicon semiconductor structure 102 under silicon dioxide layer 118, causing a minority-carrier channel to form in a region 136 of p-body 126. The minority-carrier channel has excess electrons and will therefore conduct electric current. Consequentially, current will flow in a lateral direction 138 through silicon semiconductor structure 102, from drain n+ region 132 to source n+ region 130, when VGS exceeds a threshold value and VDS is a positive value. The current can encounter substantial resistance, however, in drift region 134 due to relatively light n-type dopant concentration in n-well 124.